A mold for molding a tire is manufactured by casting using a plaster casting mold in general and is used for tire molding in a vulcanization process. During this tire molding, in order to spread rubber of the tire to every corner of the mold, an air trap between the mold and the tire needs to be eliminated. Thus, a vent hole (air vent hole) is formed in the mold so as to discharge an air between the mold and the tire through the vent hole. Moreover, in order to reliably discharge the air, each of vent holes having a diameter of 0.6 to 1.6 mm is formed in several hundreds to several thousands spots in a pair of molds, for example.
The vent hole is usually formed in the mold by drilling machining or electric discharge machining. Thus, formation of the large number of vent holes takes a lot of labor, and a formable diameter and length of a vent hole is limited. On the other hand, a vent hole forming method for easily forming a vent hole in a mold by a wire cast in a cast metal of the mold has been known (see Patent Literature 1).
In this prior-art method, the vent hole is formed by withdrawing the wire from the cast metal after the cast metal is cast in a casting mold in which the wire is disposed. However, the wire is fixed in the cast metal in a bent and deformed state by a pressure received from a molten metal in some cases. In this case, since the vent hole is also bent, accuracy of the vent hole formed in the mold deteriorates. Moreover, if the vent hole is clogged, clogging of the vent hole is solved by an elongated tool (a drill or the like). At that time, the tool stops in the middle of the vent hole or the tool is broken during insertion into the vent hole in some cases.